Conventionally, transmission power of a portable telephone is controlled at an optimal value up to the maximum transmission power Pmax (for example 25 dBm) in accordance with the position of the portable telephone, for example, so as to allow communication with a base station and not to hinder communication of other portable telephone with the base station. The amount of control per one slot of the transmission power Px is determined to be ±1 dB, ±2 dB, ±3 dB or the like, and accuracy of the amount of control ΔP is also determined to be 1 dB±0.5 dB, 2 dB±1 dB, 3 dB±1.5 dB, or the like.
Control of the transmission power of the portable telephone is realized by adjusting a control signal value Vc of a variable gain block. The portable telephone contains a storage unit, and in the storage unit, a table representing relation between transmission power set value Pout and the control signal value Vc when temperature T is 25° C. is stored, as shown in FIG. 7A. For example, assume that an instruction is issued from the base station to increase the transmission power by 1 dB, when the control signal value V2 is applied to the variable gain block and transmission is performed with transmission power set value Pout=23 dBm. In this case, the transmission power set value Pout is set to 24 dBm, by increasing the control signal value Vc to V1. The threshold power set value Ps is set to 24 dBm, and it is inhibited to set the transmission power set value Pout to be higher than the threshold power set value Ps.
When T=25° C., the transmission power set value Pout is equal to the actual transmission power Px. When the temperature T increases, however, the actual transmission power Px decreases even when the control signal value Vc is kept constant, as shown in FIG. 8. When the temperature T lowers, the actual transmission power Px increases, even when the control signal value Vc is kept constant. The reason for this is that the gain of the transmission system varies dependent on the temperature T.
In the example shown in FIG. 8, when T=40° C., by correcting the control signal value Vc to Vc+α, it is possible to compensate for the decrease in the transmission power Px. Therefore, conventionally, to the control signal value Vc read from the storage unit, a correction value a in accordance with the temperature T=40° C. is added as shown in FIG. 7B, and the corrected control signal value Vc+α is applied to the variable gain block, whereby change in transmission power Px resulting from the change in temperature is suppressed.
In the conventional method of power control, however, when power control (ΔP=1 dB) based on the instruction from the base station and power correction (ΔP′) in accordance with the temperature T are performed simultaneously (time t1), the power control amount ΔP+ΔP′ may undesirably exceed the accuracy specification (1 dB±0.5 dB), as shown in FIG. 9.